Synthetic aperture radar (SAR) is a well known technique for developing radar imagery with excellent two-dimensional (2D) resolution. This is typically done by flying an airplane over the ground to be mapped, and successively transmitting a sequence of radar pulses. With the forward motion of the airplane, each successive radar pulse is transmitted from a position a little further along on the flight path, thus simulating a very long radar array.
Synthetic aperture radar is now a mature technique used to generate radar images in which fine detail can be resolved, and provides unique capabilities as an imaging tool. Because it provides its own illumination (the radar pulses), it can image at any time of day or night, regardless of sunlight illumination. And because the radar wavelengths are much longer than those of visible or infrared light, SAR can also “see” through cloudy and dusty conditions that visible and infrared instruments cannot.
As the radar moves, a pulse is transmitted at each position; the return echoes pass through the receiver and are recorded in an “echo store.” Because the radar is moving relative to the ground, the returned echoes are Doppler-shifted (negatively as the radar approaches a target; positively as it moves away). Comparing the Doppler-shifted frequencies to a reference frequency allows many returned signals to be “focused” on a single point, effectively increasing the length of the antenna that is imaging that particular point. This focusing operation, commonly known as SAR processing, is now done digitally on powerful computer systems. SAR processing must correctly match the variation in Doppler frequency for each point in the image, however, and this requires very precise knowledge of the relative motion between the platform and the imaged objects (which is the cause of the Doppler variation in the first place).
Radar energy is transmitted in the form of sequential pulses, at different time instances in the flight path of the vehicle. The pulses interact with the terrain (and any object on the terrain) and a portion of the pulse energy is reflected back towards the platform and recorded by a detector. Returns from different objects arrive at different times at the detector. These time differences provide information on range, which is then used to create a final radar image.
Typically, the transmitted pulses in a SAR system are sinusoidal in nature. The detector records the reflected sinusoids by recording a complex number, whose phase component is directly related to the time-of-flight, or range, and its magnitude component is proportional to the reflected energy. Thus, data captured in a SAR system is complex in nature, with properties that are unique with respect to data captured in systems from other imaging modalities.
FIG. 1 is an exemplary SAR image capture system according to the prior art. In block 10, a pulse generator generates chirps to be emitted from the SAR antenna. Before the chirps are emitted by a transmitter in block 12, however, they are frequency modulated in block 11. A receiver receives chirp echoes (i.e., reflections of the frequency modulated chirps from objects in the area being imaged), and block 13 subsequently performs SAR phase history data preprocessing. The data is then digitized in block 14. Block 15 may then reconstruct the two-dimensional SAR phase history data by performing such steps as pulse compression, Doppler shift corrections, and the like. A two-dimensional (2D) SAR image may then be obtained in block 16 by taking the inverse Fourier transform of the reconstructed phase history data.
Generally, return signals from the transmitted pulses are sampled in the airplane and either processed on board for immediate exploitation or stored or transmitted for processing at another site. The processing is computationally expensive, employing such techniques as FFT, inverse FFT, or correlation on vast amounts of data. These operations require vast processing power and storage. FIG. 2A, for example, illustrates the formation of SAR image 206 by taking the inverse FFT (IFFT) 204 of SAR phase history data 202. Consequently, SAR phase history data 202 may be obtained by taking the FFT 208 of SAR image 206. Furthermore, a compressed SAR image 216 may be obtained by taking the IFFT 214 of compressed SAR phase history data 212, whereby compressed SAR phase history data 212 may be obtained by taking the FFT 218 of compressed SAR image 216. Current methods of SAR phase history data compression are discussed in A Review of Current Raw SAR Data Compression Techniques by A. El Boustuni et al.
Technology trends in the field of SAR indicate that SAR system designs are continuously pushing the envelope for increases in area coverage and resolution. These trends imply massive amounts of collected data, which in turn, stress the ability to store collected SAR data and rapidly disseminate SAR data. Furthermore, SAR data transmission is undesirably time-consuming for customers who are only interested in a particular target in the scene, as opposed to the entire scene, or customers who aren't interested in a high resolution image of the scene.